Usually, linear olefin resins such as polypropylene and polyethylene have so far been used for an ink reservoir for a writing instrument, and a polypropylene-made ink reservoir is used as an ink reservoir also for a ballpoint pen using an oil-base ink.
This is because of the reasons that a large amount of an ink is filled into an ink reservoir in a felt-tip pen using an oil-base ink and a solvent having low volatility of a vapor pressure of 0.1 mm Hg or less and low hygroscopicity is used as a solvent for a conventional oil-base ink for a ballpoint pen, so that volatilization of the solvent and absorption of moisture are not specifically regarded as problems and that it is not meaningful to change a polypropylene-made ink reservoir having a merit in terms of a cost.
On the other hand, an amide base resin for a gas barrier for preventing oxygen, carbon dioxide and moisture from coming in from the outside and polyethylene or polypropylene for preventing moisture from penetrating into the above resin are used for a laminate reservoir having a conventional laminated structure.
Usually, the above amide base resin for a gas barrier and resins such as polyethylene and polypropylene are not compatible between the polymers themselves, and therefore in general, both resins have so far been stuck together by providing an adhesive layer. The existing situation is that no methods other than the above method are available excluding a method in which a metal tube is used.
On the other hand, the present inventors have developed an ink for a writing instrument in which a solvent selected from the group consisting of alcohols having 4 or more carbon atoms in a molecule, polyhydric alcohols and glycol monoethers is used as a principal solvent for the ink and which is excellent in a writing performance and a drying property of the drawn lines and causes no strike-through.
However, when a polypropylene-made ink reservoir is charged with an ink containing the above solvent, a problem is brought about in that it is unsatisfactory for permeability of the solvent and absorption of moisture and inferior in a storing property of the ink with the passage of time.
Further, inks for a writing instrument including an oil-base ink and a water-base ink each having a composition in which various solvents are combined have been developed in recent years, and an ink reservoir for a writing instrument which is excellent in solvent resistance and moisture-absorption resistance has been increasingly desired.
In the case of a reservoir in which are provided an adhesive layer for sticking together the layers of an olefin base resin used for preventing an influence exerted from the outside as is the case with a laminate reservoir represented by a bottle for mayonnaise and an amide base resin having properties which are completely different from those of the olefin base resin, the solvent described above which is held in a linear olefin resin exerts an effect on absorption of moisture to a large extent under a humid condition, and therefore problems reside in that the amide base resin layer is swollen by moisture and that the amide base resin layer is deteriorated as the solvent permeates. Further, a problem resides in that if an adhesive layer for tightly adhering the resin layers of different kinds is present, moisture and a solvent are accumulated in a part thereof because of its affinity with both layers so that the part is swollen or lost transparency.
In light of the conventional problems described above, the present invention intends to solve them, and an object thereof is to provide an ink reservoir for a writing instrument which is sufficiently satisfactory for permeability of a solvent and absorption of moisture even under a wide variety of the environment and which is excellent in a storing property of the ink with the passage of time.